The invention relates to recycling dry fines that are generally difficult to hydrate into a main product stream. More particularly, the invention relates to reprocessing fines generated in the production of fluid absorbent polymers and copolymers.
Water-swellable polymers and copolymers are well known for their use as absorbents for aqueous fluids in diapers, sanitary products and the like. Certain of these polymers, for example those prepared from monomers of acrylic acid or methacrylie acid or their respective alkali metal or ammonium salts and typically lightly crosslinked with a crosslinking monomer having at least two polymerizable double bonds, exhibit exceptionally high capacity for adsorbing liquids and are further characterized as having good physical integrity in processing and use.
These water swellable polymers/copolymers are often employed in a particulate form of a desired particle size that effectively promotes contact with the fluid to be absorbed. In the production of acrylic acid-based copolymers by the gel formation method, a significant portion of "fines" material, that particulate material less than about 75 mesh (200 micrometers), is typically generated from the process of manufacturing the absorbent product. These processes generally include, after the gel polymer or copolymer gel is formed, a number of drying, gel breakup and grinding unit operations until an optional amount of product of a final acceptable particle size range is achieved. In the course of the process, 8-11 percent by weight of the final product may be fines, that is, particulate polymer that is finer than the desired minimum size suitable for the intended end-use of the polymer.
Initially, users employed the entire dry product, including fines, in their absorbent products. It was soon discovered, however, that the inclusion of fines resulted in lower product performance. One difficulty that often occurs when fine particles are initially contacted with an aqueous fluid is a "gel blocking" phenomenon. Upon initial hydration of a tightly packed mass of fines, only the outside layer is wetted because the fines form such a dense polymeric network that neither capillary action nor diffusion will permit penetration of the fluid into uniform contact with the interior particles. The result is a substantially reduced overall capacity of the absorbent polymer to absorb and hold aqueous fluids. In addition, for some products such as diapers, the fines material may sift from the product.
An initial solution to the fines problem was simply to screen the fines from the product. The resulting fines were stored as off-specification product with the intention of recycling the fines into the process or reprocessing them into larger sized particles through agglomeration. However, attempts at recycling the fines into the process have heretofor proved generally unsuccessful, requiring significant additional processing steps and equipment. A major difficulty with the fines particles is that they are extremely difficult to rewet for uniform blending into the main product stream.
In U.S. Pat. No. 4,950,692 superabsorbent polyacrylate fines are rehydrated to gel form by agitating for relatively long periods of time, typically one-half to one hour, followed by blending with the main gel product stream or drying and then blending with the dry product. In U.S. patent application Ser. No. 07/407,840, fines are rehydrated by rewetting under high shear conditions. While residence times for rewetting the fines are greatly reduced over U.S. '692, the recycling process does require the introduction of relatively high performance equipment into the process.
A number of workers have attempted to agglomerate fines to produce a larger size particulate for reintroduction into the product stream. These agglomeration techniques generally involved treating the fines with water or other binding agent in an environment such as a fluidized bed. The difficulty with this approach is that these processes fail to produce a product that is sufficiently bound together to survive forming into finished products without attriting and recreating the objectionable fine material, either in the process for making the aqueous fluid absorbents or in the customer's plant or product.
Thus, in view of the difficulties of the prior efforts to recycle aqueous fluid absorbent polymer fines, it would be desirable to provide a process that recycles fines into a main product stream of polymer/copolymer such that the finished product absorbent capacity and particulate integrity are equivalent to the material normally produced of a desired particle size. Such a process should not add significant processing steps or processing time.